Method and device for high density data storage

ABSTRACT

The present invention is directed to a method for producing a data storage medium on a surface of a substrate for storing data in the form of topographic features. The method comprises a first step wherein a crosslinking agent containing at least three alkyne groups is deposited on the surface of the substrate. In a second step the deposited cross linking agent is cured so as to obtain the data storage medium in the form of a crosslinked polymeric layer on the surface of the substrate. The invention is further directed to a data storage medium obtained by this method and a data storage device comprising this data storage medium.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. National-Stage entry under 35 U.S.C. §371based on international Application No. PCT/IB2009/054536, filed Oct. 15,2009, which was published under PCT Article 21 (2) and which claimspriority to European Patent Application No. 08105633.5, filed Oct. 23,2008.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the field of high-density data storageand more specifically to a data storage medium, a data storage system,and a data storage method.

2. Description of Related Art

Current data storage methodologies operate in the 0.1-10 μm regime. Inan effort to store ever more information in ever-smaller spaces, datastorage density has been increasing. In an effort to reduce powerconsumption and increase the speed of operation of integrated circuits,the lithography used to fabricate integrated circuits is pressed towardsmaller dimensions and denser imaging. As data storage size increasesand density increases and integrated circuit densities increase, thereis a developing need for compositions of matter for the storage mediathat operate in the nanometer regime.

A storage device for storing data based on the atomic force microscope(AFM) principle is disclosed in “The millipede—more than 1,000 tips forfuture AFM data storage” by P. Vettiger et al., IBM Journal ResearchDevelopment, Vol. 44, No. 3, March 2000. The storage device has a readand write function based on a mechanical x-, y-scanning of a storagemedium with an array of probes each having a tip. The probes operate inparallel with each probe scanning, during operation, an associated fieldof the storage medium. The storage medium comprises a polymer layer. Thetips, which each have an apex diameter between 5 nm to 20 nm, are movedacross the surface of the polymer layer in a contact mode. The contactmode is achieved by applying small forces to the probes so that the tipsof the probes can touch the surface of the storage medium. For thatpurpose, the probes comprise cantilevers, which carry the tips on theirend sections. Bits are represented by indentation marks ornon-indentation marks in the polymer layer. The cantilevers respond tothese topographic changes while they are moved across the surface of thepolymer layer during operation of the device in read/write mode.

Indentation marks are formed on the polymer surface by thermomechanicalrecording. This is achieved by heating a respective probe operated incontact mode with respect to the polymer layer. Heating of the tip isachieved via a heater dedicated to the writing/formation of theindentation marks. The polymer layer softens locally where it iscontacted by the heated tip. The result is an indentation, for example,having a nanoscale diameter of the tip that is used in its formation,being produced on the layer.

Reading is also accomplished by a thermomechanical concept. The probe isheated using a heater dedicated to the process of reading/sensing theindentation marks. Either a separate heater is used, which is notconnected to the tip and therefore the probe is not heated or the probeis heated but not so as to cause heating of its associated tip, that is,the heating temperature is not high enough to soften the polymer layeras is necessary for writing. The thermal sensing is based on the factthat the thermal conductance between the probe and the storage mediumchanges when the probe is moving in an indentation as the heat transportis in this case more efficient. As a consequence of this, thetemperature of the heater decreases and hence, also its electricalresistance changes. This change of resistance is then measured andserves as the measuring signal.

For such thermal probe storage applications, the media requirements aredefined by the indentation mechanics of polymers and the need to limitmedia and tip layer. Preferably, the glass transition temperature shouldbe minimized but the polymer should also be thermally stable. Thermalstability of polymers is achieved by crosslinking and using polymerswith exceptional thermal stability. Crosslinking typically produces hardmaterials that require high forces to form indents and therefore lead toincreased tip wear. With moderate write speeds, one may use highertemperatures to minimize forces and tip wear. Since the writetemperature increases with the write speed, this trade-off between heatand force is not possible for fast writing which requires operation ofthe cantilever heater element at its maximum design temperature.

Accordingly, it is desirable to provide a method of producing a datastorage medium which reconciles the conflicting requirements of highcrosslink density for media wear resistance and low glass transitiontemperature for soft writing conditions.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is provideda method of producing a data storage medium on a surface of a substratefor storing data in the form of topographic features, comprising thesteps of: (a) Deposition of a crosslinking agent on a surface of asubstrate, said crosslinking agent containing at least three alkynegroups (i.e. containing at least three carbon-carbon triple bonds); (b)curing the deposited crosslinking agent, thereby producing a modifiedsurface of the substrate, so as to obtain the data storage medium in theform of a crosslinked polymeric layer on the surface of the substrate.

According to the first aspect of the present invention, a layer of acrosslinking agent is deposited on the surface of a substrate.Simultaneous or subsequent to the deposition of the crosslinking agent,the deposited cross linking agent is cured thereby obtaining a layercontaining cured crosslinking agent. This layer exhibits a highcrosslink density. In one embodiment the substrate may be, for example,a support layer comprising a curable polymer, particularly a polymerwith alkyne groups in the polymeric backbone or in the end groups ofthis polymer. In a further embodiment the substrate may be a templatefrom which the layer of the cured crosslinking agent can be transferredto a target layer, particularly a polymeric target layer. By curing thecrosslinking agent and the layer of the crosslinking agent respectivelythe at least three alkyne groups reconfigure to form a ladder networkwhich provides a strong chemical bond between the molecules and—ifapplicable—the polymer of the support layer used as a substrate.Therefore, according to present invention a bilayered material isobtained comprising a thin, hard crust deposited on/transferred to asoft underlayer.

According to an embodiment of the first aspect of the present invention,the deposition of the crosslinking agent on the surface of the substrateis conducted by evaporation of the crosslinking agent from a source anda at least partial deposition on the surface of the substrate used as atarget. By using this technique very thin layers of polymer can bedeposited on a substrate; no dewetting phenomena during the process ofcuring can be observed (due to the instability of thin layers of athickness of, for example, 5 nm). In contrast, using other depositionmethods like spin-casting results in dewetting phenomena. Using asurfactant that lowers the surface energy of the substrate does notsolve this problem; a dewetting of the spin-casted polymer cannot beavoided.

According to a further embodiment of the first aspect of the presentinvention the crosslinking agent is selected from compounds of thestructure ZR₃ and/or ZR'₄. Z and Z′ stands for a linking moiety,particularly an aromatic linker, linking the substituents containing thealkyne groups (Z has 3 substituents R and Z′ has at least 4 substituentsR, each of the substituents R containing at least one alkyne group; thesubstituents R are covalently bound to Z and Z′, respectively).

For example Z may be a 1,3,5-substituted six membered aromatic ring or a1,2,4-substituted six membered aromatic rings. Furthermore, the linkingmoiety may contain more than one aromatic ring. For example each of theat least two rings may contain at least two substituents R. For exampleZ′ may contain 2 six membered aromatic rings which are connected via asingle bond or an alkylene- or arylene-linker or an oxygen atom. Each ofsaid 2 six membered aromatic rings of Z′ for example may be a3,5-substituted or a 3,4-substituted aromatic ring. Furthermore, threearomatic rings may be contained, each of these at least three rings withat least one substituent R containing at least one alkyne group.

Alternatively Z and Z′ may be an aliphatic linking moiety, particularlya moiety which results in a desired special alignment substituents R (Zhaving 3 substituents R and Z′ having 4 substituents R, each of thesubstituents R containing at least one alkyne group).

The substituent R of the crosslinking agent of the structure ZR₃ and/orZR'₄ stands stands randomly and independently from one another for amoiety comprising at least one alkyne group and a substituted orunsubstituted aromatic moiety or a hydrogen atom or consists of analkyne group and a substituted or unsubstituted aromatic moiety or ahydrogen atom.

Often all substituents R are identical; a structure of the crosslinkingagent being less polar leads to a crosslinking agent being easier toevaporate.

Preferably the linking moiety Z or Z′ represents

or a silicon atomwherein * denotes a bond between R and Z or Z′.

The linker L between the two aromatic rings of the linking moietypreferably represents oxygen, an alkylene or arylene moiety.Alternatively L may be a single bond between the two aromatic rings.Preferably, alkylene linkers are methylene linkers or substitutedmethylene linkers like C(CH₃)₂.

Preferably the substituent R containing the at least one alkyne groupstands randomly and independently from one another for a moietycomprising a substituted alkyne group (i.e. comprising no terminalalkyne), a meta- or para-substituted phenylene group and/or a phenylgroup. Alternatively, R stands for a moiety consisting of a substitutedalkyne group, a meta- or para-substituted phenylene moiety and/or asubstituted or unsubstituted phenyl group. Un-substituted phenyl groupsare preferred; crosslinking agents containing substituted phenyl groupsare less temperature stable. The substituent R may also contain two ormore alkyne groups and two or more meta- or para-substituted phenylenemoieties.

Most preferably the substituents R represent randomly and independentlyfrom one another

wherein * denotes a bond between R and Z or Z′.

The crosslinking agent ZR₃ and/or ZR′₄′ represents most preferably:

In a further embodiment the used crosslinking is vaporizable attemperatures below 300° C. At higher temperatures crosslinking agentscontaining alkyne groups often tend to polymerize. More preferably, thecrosslinking agent is vaporizable at temperatures below 250° C. and mostpreferably at temperatures between 150 and 200° C. At these temperaturesbetween 150 and 200° C. the ratio of the evaporated crosslinking agentenables a satisfying deposition rate.

In one example, the crosslinking agent according to embodiments of thepresent invention advantageously has a molecular weight of below about900 Daltons (for the purpose of describing the present invention,Daltons and g/mol may be used interchangeably). More preferably thecrosslinking agent has a molecular weight of between 270 and 800Daltons.

In an embodiment of the present invention, the crosslink polymeric layeris deposited on the surface of a template. According to this embodimentafter curing of the crosslinking agent the following steps are carriedout:

(c) The modified surface of the substrate (i.e. the modified surface ofthe template) is contacted with a surface of a target layer. Thereby anassembly is obtained comprising the substrate with the crosslinkedpolymeric layer and the target layer adjacent to the modified surface.In step (d) a liquid is introduced to an environment of the assemblyobtained in step (c). As a result the layer of the cured crosslinkingagent is transferred onto at least an adjacent region on the targetsurface. The template surface is chosen on account of its surfaceroughness profile and is preferably relatively defect-free. The exposedsurface of the cured cross-linking agent layer of the bilayered material(i.e. the layer of the data storage medium on top of the support layer)obtained in step (d) demonstrates the same degree of flatness as thetemplate surface that it was previously in contact with. A method wherea layer of a data storage medium is transferred from a template layer toa target layer is also described in WO 2007/113760, the disclosurecontent of which is hereby incorporated by reference.

In one example, the surface of the template used in this embodiment hasa hydrophilic character. Desirably, the template surface comprises asurface of one of: a mica substrate, a flame-annealed glass substrate, asilicon-oxide layer on a silicon substrate and a (100) surfaceperovskite substrate. It is also preferable that heating of the modifiedsurface produced in step (b) is conducted prior to step (c). It isdesirable that the liquid introduced in step (d) comprises a polarliquid.

In a further example in this embodiment the cured layer of thecrosslinking agent is obtained by deposition of a mixture of thecrosslinking agent and a comonomer in step (a). In step (b) the mixtureof deposited crosslinking agent and comonomer are cured, therebyproducing the crosslinked polymeric layer.

As a comonomer, a compound bearing at least two alkyne groups can beused. Usually the comonomer exhibits a similar thermal stabilitycompared to the crosslinking agent. For example, compounds of thegeneral structure Z″R₂ (Z″ has 2 substituents R, each of thesubstituents R containing at least one alkyne group; the substituents Rare covalently bound to Z″) can be used wherein R is defined as above(in compounds ZR₃ and Z′R₄). The central moiety Z″stands for a moietyconnecting two substituents R, for example, for an aromatic ring withtwo substituents R (for example in 1,3- or 1,4-position of a 6 membertwin) or a biphenyl with two substituents R (for example in4,4′-position). The ratio between the cross-linking agent and thecomonomer is preferably between 100:0 and 20:80 (mol-%). Preferably theratio of the comonomer is below 50 mol-%, more preferably below 20mol-%.

The method according to this embodiment results in a bilayered materialexhibiting sharp boundaries between the layers. The thin layer of thecured crosslinking agent is directly deposited onto a particular kind ofsurface from which it is possible to transfer the media onto a commontarget substrate. On top of the crosslinked layer for example a stablethick layer of a standard polymer can be spin-coated. The obtainedsandwich of the cured crosslinking layer and standard polymer may becured. Since the layer of the cured crosslinking agent is highlycrosslinked and the molecular weight of the standard polymer is usuallyrather high, no significant interdiffusion of the layers is observed.

In a further embodiment of the present invention the crosslinking agentis deposited on the surface of a substrate which is a support layercomprising one or more crosslinkable polymers containing alkyne groups.These alkyne groups of the crosslinkable polymers may be contained inthe polymeric back bone; alternatively the alkyne groups may also becontained in the end groups of the polymer. In this embodiment of theinvention the deposited molecules of the crosslinking agent arrive atthe surface of the support layer and enter the polymer via diffusion andfind crosslinking partners. Thereby, they locally enhance thecross-linking density which leads to a bilayered material having nosharp boundaries between the layers; a data storage medium on top of thesupport layer is obtained.

In one example the support layer contains at least onepolyaryletherketone polymer as described in US 2007/0296101 A1, which isincorporated hereby by reference. In another example the support layercomprises at least one polyimide oligomer as described in WO 2007/096359A2 which is hereby incorporated by reference. Preferably each of saidpolyaryletherketone polymers and/or polyimide oligomers has two terminalends each terminal end having two or more phenyl moieties. For examplethe following polyaryletherketone polymers described in US 2007/0296101A1 may be used:

-   -   wherein R¹ is selected from the group consisting of:

-   -   wherein R² is selected from the group consisting of:

-   -   wherein R³ is selected from the group consisting of        poly(arylacetylenes), poly(phenylethynyls),

and

-   -   wherein n is a integer from about 5 to about 50.        In a further embodiment in the polyaryletherketone polymers        described in US 2007/0296101 A1 0-30% of the number of R²        moieties in

are replaced by the following moiety:

The synthesis of polyaryletherketone polymers containing these moiety iscarried out as described in US 2007/0296101 A1 using a mixture of thestarting materials for the R² moieties.

In a further example the support layer contains at least one polyimideoligomer. For example the following polyimide oligomers described in WO2007/096359 A2 may be used:

-   -   wherein R′ is selected from the group consisting of:

-   -   wherein R″ is selected from the group consisting of:

-   -   wherein n is a integer from about 5 to about 50.        As a further example the following polyimide oligomers described        in WO 2007/096359 A2 may be used:        E-R′-(A₁-A₂-A₃- . . . -A_(N)-)R″—R′-E    -   wherein E is

-   -   wherein each of A₁, A₂, A₃ . . . A_(N) is independently selected        from the group consisting of:

-   -   wherein R′ and R″ are as defined above;    -   wherein R′″ is

-   -   and wherein N is an integer greater than or equal to 2.

In an example the polymer contained in the support layer (in embodimentswhere the substrate is a template or a polymeric crosslinkable polymerrespectively) has a high temperature stability and low glass transitiontemperature (TG). Preferably the glass transition temperature is lessthan 220° C., more preferably between 100° C. and 180° C., mostpreferably between 100° C. and 150° C. The glass transition temperatureof the layer of the cured cross-linking agent is usually higher than theglass transition temperature of the polymer of the support layer.Preferably the difference between both glass transition temperatures isat least 50° C.

In a further embodiment the curing of the crosslinking agent isconducted at a temperature between 330° C. and 450° C., preferablybetween 350 and 450° C. and most preferably between 380° C. and 430° C.Usually steps (a) and (b) of the method of the present invention takeplace simultaneously.

As the evaporation of the crosslinking agent is carried out at a lowertemperature than the temperature for the curing of the crosslinkingagent also a re-evaporation of the already deposited crosslinking agentoccurs at the surface of the substrate. However, altogether a growth ofthe layer of the cured crosslinking agent can be observed.

For the curing of the crosslinking agent further components may bepresent at the surface of the substrate or in the area of the substratebeing adjacent to the surface (i.e. the support layer comprisingpolymers containing alkyne groups and the template from which the layerof the cured crosslinking agent can be transferred to a target layer,respectively). These further components include activators to start apolymerization reaction (e.g. radical starters and photoactivators) andmolecules or compositions to facilitate processing (e.g. adhesionenhancers, antifoam agents and stabilizers).

In a further embodiment the thickness of the layer of the curedcrosslinking agent is at least 5 nm. For use as a data storage mediumusually a thickness of between 5 and 10 nm is sufficient.

The deposition of the crosslinking agent on the surface of the substratecan be carried out also by other techniques. Besides evaporation from asource and deposition on a target, techniques like plasma deposition canalso be used. However, usage of plasma deposition may result in anuncontrolled nature of the crosslink reaction and difficulties in thecontrol of the overall process.

According to a second aspect the present invention also extends to adata storage medium for storing data in the form of topographic featuresproduced according to an embodiment of the method aspect of the presentinvention.

According to a third aspect the present invention extends to a datastorage device incorporating a data storage medium according to thesecond aspect and further comprising at least one probe for writingand/or reading the data stored in the data storage medium.

Features of one aspect of the present invention may be applied to anyother aspect and vice versa.

BRIEF DESCRIPTION OF DRAWINGS

The features of the invention are set forth in the appended claims. Theinvention itself, however, will be best understood by reference to thefollowing detailed description of an illustrative embodiment when readin conjunction with the accompanying drawings, wherein:

FIGS. 1A through 1C illustrate the structure and operation of a tipassembly for a data storage device including the data storage mediumaccording to the embodiments of the present invention;

FIG. 2 is an isometric view of a local probe storage array including thedata storage medium according to the embodiments of the presentinvention;

FIG. 3 shows schematically the setup for evaporation and deposition ofthe cross-linking agent;

FIG. 4 shows an example of the data storage medium;

FIG. 5 and FIG. 6 show a grid of bits indented in a low crosslinkedpolymer and a low crosslinked polymer with a layer of a crosslinkingagent deposited and cured on the surface of this polymer, respectively;and

FIG. 7 shows the temperature-force relation for bits indented in a lowcrosslinked polymer with different depth.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1A through 1C illustrate the structure and operation of a tipassembly 100 for a data storage device including the data storage mediumaccording to the embodiments of the present invention. In FIG. 1A, probetip assembly 100 includes a U-shaped cantilever 105 having flexiblemembers 105A and 105B connected to a support structure 110. Flexing ofmembers 105A and 105B provides for substantial pivotal motion ofcantilever 105 about a pivot axis 115. Cantilever 105 includes anindenter tip 120 fixed to a heater 125 connected between flexing members105A and 105B. Flexing members 105A and 105B and heater 125 areelectrically conductive and connected to wires (not shown) in supportstructure 110. In one example, flexing members 105A and 105B andindenter tip 120 are formed of highly-doped silicon and have a lowelectrical resistance, and heater 125 is formed of lightly doped siliconhaving a high electrical resistance sufficient to heat indenter tip 120,in one example, to between about 100° C. and about 500° C. when currentis passed through heater 125. The electrical resistance of heater 125 isa function of temperature.

Also illustrated in FIG. 1A is a storage medium (or a recording medium)130 comprising a substrate 130A, and a support layer 130C. In oneexample, support layer 130C is a polyaryletherketone resin layer. In oneexample, support layer 130C has a thickness between about 10 nm andabout 500 nm. On top of the support layer 130C a layer of the curedcross-linking agent 130B is shown.

Turning to the operation of tip assembly 100, in FIG. 1A, an indentation135 is formed in cured crosslinking agent layer 130B by heating indentertip 120 to a writing temperature TW by passing a current throughcantilever 105 and pressing indenter tip 120 cured crosslinking agentlayer 130B. Heating indenter tip 120 allows the tip to penetrate thecured crosslinking agent layer 130B forming indentation 135, whichremains after the tip is removed. In a first example, the curedcrosslinking agent layer 130B is heated by heated indenter tip 120, thetemperature of the indenter tip being not greater than about 500° C., toform indentation 135. In a second example, the cured crosslinking agentlayer 130B is heated by heated indenter tip 120, the temperature of theindenter tip being not greater than about 400° C., to form indentation135. In a third example, the cured crosslinking agent layer 130B isheated by heated indenter tip 120, the temperature of the indenter tipbeing between about 200° C. and about 500° C., to form indentation 135.In a fourth example, the cured crosslinking agent layer 130B is heatedby heated indenter tip 120, the temperature of the indenter tip beingbetween about 100° C. and about 400° C., to form indentation 135. Asindentations 135 are formed, a ring 135A of cured crosslinking agent isformed around the indentation. Indentation 135 represents a data bitvalue of “1”, a data bit value of “0” being represented by an absence ofan indentation. Indentations 135 are nano-scale indentations (several toseveral hundred nanometers in width).

FIGS. 1B and 1C illustrate reading the bit value. In FIGS. 1B and 1C,tip assembly 100 is scanned across a portion of cured crosslinking agentlayer 130B. When indenter tip 120 is over a region of cured crosslinkingagent layer 130B not containing an indentation, heater 125 is a distanceD1 from the surface of the cured crosslinking agent layer 130B (see FIG.1B). When indenter tip 120 is over a region of cured crosslinking agentlayer 130B containing an indentation, heater 125 is a distance D2 fromthe surface of the cured crosslinking agent layer (see FIG. 1C) becausethe tip “falls” into the indentation. D1 is greater than D2. If heater125 is at a temperature TR (read temperature), which is lower than TW(write temperature), there is more heat loss to substrate 130A whenindenter tip 120 is in an indentation than when the tip is not. This canbe measured as a change in resistance of the heater at constant current,thus “reading” the data bit value. It is advantageous to use a separateheater for reading, which is mechanically coupled to the tip butthermally isolated from the tip.

“Erasing” (not shown) is accomplished by positioning indenter tip 120 inclose proximity to indentation 135, heating the tip to a temperature TE(erase temperature), and applying a loading force similar to writing,which causes the previously written indent to relax to a flat statewhereas a new indent is written slightly displaced with respect to theerased indent. The cycle is repeated as needed for erasing a stream ofbits whereby an indent always remains at the end of the erase track. TEis typically greater than TW. The erase pitch is typically on the orderof the rim radius. In a first example, the cured crosslinking agentlayer 130B is heated by heated indenter tip 120, the temperature of theindenter tip is not greater than about 500° C., and the erase pitch is10 nm to eliminate indentation 135. In a second example, the curedcrosslinking agent layer 130B is heated by heated indenter tip 120, thetemperature of the indenter tip is not greater than about 400° C., andthe erase pitch is 10 nm to eliminate indentation 135. In a thirdexample, the cured crosslinking agent layer 130B is heated by heatedindenter tip 120, the temperature of the indenter tip is between about200° C. and about 400° C., and the erase pitch is 10 nm to eliminateindentation 135. In a fourth example, the cured crosslinking agent layer130B is heated by heated indenter tip 120, the temperature of theindenter tip is between about 200° C. and about 500° C., and the erasepitch is 10 nm to eliminate indentation 135.

FIG. 2 is an isometric view of a local probe storage array 140 includingthe data storage medium according to the embodiments of the presentinvention. In FIG. 2, local probe storage array 140 includes substrate145 having a polymeric support layer with a layer of the cured crosslinking agent (not shown) on top of it (polymeric layer 150), which actsas the data-recording layer. In one example, polymeric layer 150 has athickness between about 10 nm and about 500 nm and a root mean squaresurface roughness across a writeable region of polymeric layer 150 ofless than about 1.0 nm across the polymeric layer 150. Positioned overpolymeric layer 150 is a probe assembly 155 including an array of probetip assemblies 100. Probe assembly 155 may be moved in the X, Y and Zdirections relative to substrate 145 and polymeric layer 150 by anynumber of devices as is known in the art. Switching arrays 160A and 160Bare connected to respective rows (X-direction) and columns (Y-direction)of probe tip assemblies 100 in order to allow addressing of individualprobe tip assemblies. Switching arrays 160A and 160B are connected to acontroller 165 which includes a write control circuit for independentlywriting data bits with each probe tip assembly 100, a read controlcircuit for independently reading data bits with each probe tip assembly100, an erase control circuit for independently erasing data bits witheach probe tip assembly 100, a heat control circuit for independentlycontrolling each heater of each of the probe tip assembles 100, and X, Yand Z control circuits for controlling the X, Y and Z movement of probeassembly 155. The Z control circuit controls a contact mechanism (notshown) for contacting the cured polyaryletherketone resin layer 150 withthe tips of the array of probe tip assemblies 100.

During a write operation, probe assembly 155 is brought into proximityto polymeric layer 150 and probe tip assemblies 100 are scanned relativeto the polymeric layer 150. Local indentations 135 are formed asdescribed supra. Each of the probe tip assemblies 100 writes only in acorresponding region 170 of polymeric layer 150. This reduces the amountof travel and thus time required for writing data.

During a read operation, probe assembly 155 is brought into proximity topolymeric layer 150 and probe tip assemblies 100 are scanned relative tothe polymeric layer 150. Local indentations 135 are detected asdescribed supra. Each of the probe tip assemblies 100 reads only in acorresponding region 170 of polymeric layer 150. This reduces the amountof travel and thus the time required for reading data.

During an erase operation, probe assembly 155 is brought into proximityto polymeric layer 150, and probe tip assemblies 100 are scannedrelative to the polymeric layer 150. Local indentations 135 are erasedas described supra. Each of the probe tip assemblies 100 reads only in acorresponding region 170 of cured polymeric layer 150. This reduces theamount of travel and thus time required for erasing data.

Additional details relating to data storage devices described supra maybe found in the articles “The Millipede—More than one thousand tips forfuture AFM data storage,” P. Vettiger et al., IBM Journal of Researchand Development. Vol. 44 No. 3, May 2000 and “TheMillipede—Nanotechnology Entering Data Storage,” P. Vettiger et al.,IEEE Transaction on Nanotechnology, Vol. 1, No, 1, March 2002. See alsoUnited States Patent Publication 2005/0047307, Published Mar. 3, 2005 toFrommer et al. and United States Patent Publication 2005/0050258,Published Mar. 3, 2005 to Frommer et al., both of which are herebyincluded by reference in their entireties.

FIG. 3 illustrates the evaporation set-up for the evaporation of thecrosslinking agent. The set-up comprises two thermal conducting plates220, 230 that can be heated separately up to 420° C. The plates areplaced parallel to each other and separated, for example, by a distanceof 4 cm. The source 200 of the evaporation process is clamped to thebottom plate 220. For this purpose a thin film of the crosslinkingagent, for example the crosslinking agent1,3,5-Tris(4-(phenylethynyl)phenyl)benzene (structure II) is spin-castfrom the solution onto a silicon wafer. Facing the source 200, thetarget wafer 210 is attached to the top plate 230. A shutter 240 isplaced between the two plates which can effectively initiate or stop thedeposition of the source material onto the target. The set-up is placedin a high vacuum chamber (not shown).

The temperature calibration of the evaporation process was performed bykeeping the target at room temperature. Before opening the shutter 240,the source temperature was raised to 120° C. in order to evaporate anyabsorbed molecules of water or other contaminants. It was found thatefficient evaporation of the crosslinking agent, particularly of1,3,5-Tris(4-(phenylethynyl)phenyl)benzene (structure II), is obtainedat temperatures between 150° C. and 200° C. The average thickness of thefilm deposited on the target wafer after 10 minutes of evaporation was23 nm as measured by ellipsometry.

In a second experiment, the target was maintained at 400° C. during theentire evaporation. The objective was to initiate the crosslinkingreaction as soon as the crosslinking agent reaches the target. Becauseof this high temperature, a part of the molecules of the crosslinkingagent re-evaporated from the target and a thinner film compared to theexample before was obtained at the same evaporation conditions as in thefirst experiment. After 10 minutes of evaporation the thickness was 1.5nm as measured by ellipsometry. A layer with a thickness of 5-10 nm wasobtained after an evaporation time of 30 to 70 minutes.

In a third experiment a spin-cast film with a thickness of 134 nm(measured by ellipsometry) of a low crosslinked high temperature polymerserved as a target (e.g. the polyaryletherketone polymer obtained from4,4′-difluorobenzophenone, resorcinol and3,5-bis(4-(phenylethynyl)-phenyl)phenol can be used—this polymer isdescribed in US 2007/0296101 A1). After evaporation of the crosslinkingagent, the overall thickness of the target was again 134 nm as measuredby ellipsometry. However, a detailed analysis revealed a layeredstructure with a top layer having a thickness of 9 nm (measured byellipsometry) comprising a cured mixture of1,3,5-Tris(4-(phenylethynyl)phenyl)benzene (structure II) and substratepolymer (the target was maintained at 400° C. in this experiment). Theexistence of a cured mixture can be proven by ellipsometry: the measuredvalue of the refractive index is between the values for pure curedcrosslinking agent and pure cured low crosslinked polymer. Thereforethis experiment shows that the crosslinking agent is free to diffuse andto react locally with the polymer, thereby increasing the crosslinkeddensity. The hardness of the obtained media correlates with thecrosslinked density.

FIG. 4 shows an example of the data storage medium obtained by aforesaidexperiments. This data storage medium comprises a substrate (145) madeof silicon or another material and a not crosslinked or slightlycrosslinked sublayer (151) for example a polyaryletherketone polymerlayer (usually the fraction of crosslinked monomers in the backbone is<10%). The data storage medium further comprises a top layer (152) whichis highly cross-linked and contains a cured crosslinking agent (forexample 1,3,5-Tris(4-(phenylethynyl)phenyl)benzene). The thickness ofthe top layer is usually 5-10 nm.

FIGS. 5 and 6 show a n array of bits which has been written onto thesurface of a low crosslinked polymer (FIG. 5) and a layered structureobtained from a low crosslinked polymer with a layer of crosslinkingagent deposited on top of it (FIG. 6), respectively.

The low crosslinked polymer shown in FIG. 5 is the polyaryletherketonepolymer described in the preceding paragraph. The layered structure usedfor the written grid of bits shown in FIG. 5 is also described in thepreceding paragraph.

For writing the grid of bits a millipede set-up was used. Each one ofthe four blocks in both figures was written at a different temperature,respectively from bottom to top, 100° C., 230° C., 367° C. and 500° C.Within each block, the force is increased after three lines,respectively, 85 nN, 105 nN, 125 nN and 145 nN.

FIG. 7 shows the temperature-force relation for bits with a given depthof 1 nm (“1”), 2 nm (“2”), 3 nm (“3”) and 4 nm (“4”) which have beenwritten onto the surface of a low crosslinked polymer (continuos lines)and a layered structure obtained from a low crosslinked polymer with alayer of crosslinking agent deposited on top of it (dashed lines),respectively. The polymer/layered structure used for this experiment isidentical to the polymer/layered structure described in the precedingparagraph.

The displacement of the curves towards higher forces for the sampleshaving a top layer of cured crosslinking agent clearly indicates theincrease of the hardness as expected. In fact, the increasing ofcrosslink sides locally reduced mobility of the polymer chains. Thus,the glass transition temperature increases and locally, a hardermaterial is formed. By this method, it is expected to obtain media whichexhibit a gradient of hardness from the top because the supply ofcrosslink agents is performed through a diffusion process. By changingthe evaporation time, and eventually, the temperature of the target, onecan obtain media with different diffusion lengths. In this way, it ispossible to tune the properties of the media in order to optimize thewriting conditions, the bit retention and the wear of the tip.

Synthesis of polymers and crosslinking agents:

The crosslinking agents can be synthesized according to the exemplarysynthesis described in U.S. Pat. No. 6,713,590 B2. Moreover,1,3,5-tris[4-(phenylethynyl)phenyl]benzene can be synthesized accordingto S. V. Lindeman et al., Russian Chemical Bulletin C/C ofIzvestiia-Akademiia Nauk Seriia Khimicheskaia 1994, 43, 1873 oraccording to Connor et al., Adv. Mater. 2004, 16, 1525.

Polyaryletherketone polymers are synthesized as described in US2007/0296101 A1. Polyimide oligomers are synthesized as described in WO2007/096359 A2.

While at least one exemplary embodiment has been presented in theforegoing detailed description of the invention, it should beappreciated that a vast number of variations exist. It should also beappreciated that the exemplary embodiment or exemplary embodiments areonly examples, and are not intended to limit the scope, applicability,or configuration of the invention in any way. Rather, the foregoingdetailed description will provide those skilled in the art with aconvenient road map for implementing an exemplary embodiment of theinvention, it being understood that various changes may be made in thefunction and arrangement of elements described in an exemplaryembodiment without departing from the scope of the invention as setforth in the appended claims and their legal equivalents.

The invention claimed is:
 1. A method for producing a data storagemedium on a surface of a substrate for storing data in the form oftopographic features, comprising the steps of: (a) depositing acrosslinking agent on the surface of the substrate, said crosslinkingagent comprising at least three alkyne groups; and (b) curing thedeposited crosslinking agent, thereby producing a modified surface ofthe substrate, so as to obtain the data storage medium as a crosslinkedpolymeric layer on the surface of the substrate.
 2. The method of claim1 wherein step (a) includes evaporating the crosslinking agent to form avapor and depositing the vapor of the crosslinking agent on the surfaceof the substrate.
 3. The method claim 1 wherein: the crosslinking agenthas the structure ZR₃ and/or Z′R₄; and Z and Z′ has the relevance of anaromatic and/or an aliphatic linking moiety and R represents randomlyand independently from one another a moiety comprising an alkyne groupand a substituted or unsubstituted aromatic moiety and/or a hydrogenatom at the terminal carbon atom of the alkyne.
 4. The method of claim 3wherein the linking moiety Z or Z′ represents

or a silicon atom; wherein * denotes a bond between R and Z or Z′ andwherein L represent O, CH₂, C(CH₃)₂ an arylene moiety or a single bondbetween the two aromatic rings.
 5. The method of claim 3 wherein Rrepresents randomly and independently from one another a moietycomprising a substituted alkyne group, a meta- or para-substitutedphenylene moiety and/or a phenyl group.
 6. The method of claim 5 whereinR represents randomly and independently from one another

wherein * denotes a bond between R and Z or Z′.
 7. The method of claim 1wherein the cross linking agent is vaporizable at temperatures <300° C.8. The method of claim 1, wherein the cross linking agent has amolecular weight below about 900 Daltons.
 9. The method of claim 1,wherein: the substrate is a template; and subsequent to step (b) thefollowing steps are carried out: (c) contacting the modified surfaceproduced in step (b) with a surface of a target layer thereby obtainingan assembly comprising the substrate with the crosslinked polymericlayer and the target layer adjacent to the modified surface; and (d)transferring the target layer with the data storage medium thereon ontoat least an adjacent region on the target surface by introducing aliquid to an environment of the assembly obtained in step (c), whereinthe target layer of steps (c) and (d) is a support layer for thecrosslinked polymeric layer and the data storage medium is obtained on asurface of the support layer.
 10. The method claim 9 wherein the surfaceof the template is a hydrophilic surface and the liquid introduced instep (d) comprises a polar liquid.
 11. The method claim 9, wherein thesurface of the template comprises a surface selected from a groupconsisting of a mica substrate, a flame annealed glass substrate, asilicon oxide layer on a silicon substrate, and a (100) surfaceperovskite substrate salt layer.
 12. The method of claim 9, wherein: instep (a) a mixture of the crosslinking agent and a comonomer isdeposited on the surface of the substrate said comonomer containing atleast two alkyne groups; and in step (b) the mixture of the depositedcross linking agent and the deposited comonomer are cured therebyproducing the crosslinked polymeric layer and obtaining a modifiedsurface.
 13. The method of claim 1 wherein: the substrate is a supportlayer comprising one or more crosslinkable polymers containing alkynegroups, and in step (b) the deposited cross linking agent and thecrosslinkable polymer are cured thereby producing a crosslinkedpolymeric layer so as to obtain the data storage medium on a surface ofthe support layer.
 14. The method of claim 13 wherein the support layercomprises one or more polyaryletherketone polymers and/or polyimideoligomers, each of said one or more polyaryletherketone polymers and/orpolyimide oligomers having at least two terminal ends, each terminal endhaving two or more phenylethynyl moieties.
 15. The method of claim 1,wherein step (b) is conducted at a temperature between 330° C. and 450°C.
 16. The method of claim 1, wherein the thickness of the data storagemedium is at least 5 nm.
 17. The method of claim 1, wherein step (a) isconducted by plasma deposition.
 18. A data storage medium for storingdata including in topographic features formed according to claim
 1. 19.A data storage device comprising: a data storage medium for storing dataincluding topographic features formed according to claim 11; and atleast one probe for writing and/or reading the data stored in the datastorage medium.